Toss your pears with the cinnamon, nutmeg, brown sugar, rosemary and the vanilla bean seeds in a nonstick baking dish. Nestle the reserved bean into the pieces of pear and roast for 45 minutes, or until the pear is tender and caramelized.
Discard the beans, and transfer the puree to a blender, along with the pear nectar, water, lemon juice and maple syrup to taste. Process until silky smooth. Transfer to a dish and refrigerate until cold.
Once cold, pour the puree into the ice cream maker (as per the your unit's directions). You can eat it soft out of the machine or pop into the freezer to harden up before scooping and serving.